Telemetry tracking and command satellite link

ABSTRACT

The invention relates to a telemetry, control, and ranging link of a satellite; it proposes using spread spectrum transmission for the up link, i.e. for commanding the satellite and for the ranging up signal. It proposes using subcarrier modulation for the down link, i.e. for telemetry and for the ranging down signal. This makes it possible for the up link to withstand interference and to provide co-localization; on the down link, the use of subcarrier modulation makes it possible to continue using existing equipment and known solutions.

[0001] The invention relates to satellites, and more particularly to thetelemetry, command, and distance measuring link between ground stationsand a satellite. This link is commonly referred to as the TTC link(acronym for telemetry, tracking and command) or else the TCR link (fortelemetry, command, and ranging). Such a link needs to be established inhighly reliable manner throughout all stages in the lifetime of asatellite, amongst which four main stages can be distinguished:

[0002] a stage of moving onto station, which corresponds to the periodfrom injection by the launcher until the satellite reaches its finalposition;

[0003] a stage of keeping station, which corresponds to the stage ofnominal operation of the satellite;

[0004] an emergency stage, where appropriate, corresponding to abreakdown and during which the altitude of the satellite can bemodified; and

[0005] a stage of deactivation or de-orbiting during which the satelliteis sent to a “graveyard” orbit.

[0006] The link is used:

[0007] firstly for remote command of the satellite from the earth(sending instructions that are to be executed on board the satellite),referred to below as “TC”;

[0008] and secondly for telemetry, i.e. for the satellite transmittinginformation concerning its own state (technological verification,reporting on the execution of commands given remotely, attitude data, .. . ) with telemetry being referred to below as “TM”.

[0009] It is also advantageous to use the same carriers as are used forcommand and telemetry in order to measure the distance between theground station and the satellite.

[0010] Proposals have been made to use modulation with a subcarrier onthe TTC link of a satellite; that solution is used on export satellitesand on many space agency satellites. Proposals have also been made undersuch circumstances to use the residue of the telemetry signal carrierused for telecommunications purposes as a beacon signal; such a signalthen provides pointing assistance to ground stations. For example, it ispossible to use BPSK/FM or BPSK/PM modulation on the up channel andBPSK/PM modulation on the down channel; conventional notation is beingused here where the modulation specified before the slash symbol is themodulation applied to the subcarrier, and the modulation after the slashsymbol is that which is applied to the carrier. BPSK is an acronym for“binary phase shift keying” and is modulation by shifting phase betweentwo states. FM and PM stand respectively for frequency modulation andfor phase modulation.

[0011] When such subcarrier modulation is used, one known solution formeasuring distance is as follows: the ground station transmits puresinewave tones modulating the up carrier of the command link. Thesetones are demodulated on board and then used to remodulate the telemetrycarrier on the down channel, simultaneously with the telemetrysubcarrier. Range can be deduced by the phase shift measured on thehighest frequency tone (known as the “major” tone), and ambiguity can beremoved using so-called “minor” tones which are derived from the majortone by a geometric progression; thus, the European Space Agency (ESA)standard proposes a geometric progression of ratio 1/5.

[0012] Such subcarrier modulation suffers from the following drawbacks:firstly, the modulation is not robust in terms of resistance tointerference; secondly such modulation is poorly adapted toco-localization, where co-localization consists in locating a pluralityof satellites using the same frequency band. This is the normalsituation for TC signals in BPSK/FM; these signals are used massively onexport, and present a large bandwidth, about 800 kilohertz (kHz) for FMmodulation having an excursion of +400 kHz. Co-localization of aplurality of satellites requires this large bandwidth (plus the bandrequired by the selectivity of TC receivers) to be multiplied by thenumber of satellites. Co-localization of a plurality of satellites canthus require a frequency band that is greater than that proposed insatellite frequency band allocation. Thus, for a constellation of 11satellites, co-localization using TC FM modulation with a subcarrierrequires a frequency band of about 15 megahertz (MHz), making anallowance for the conventional selectivity of TC receivers.

[0013] It is also known to use spread spectrum modulation for the TTClink of a satellite; this solution is implemented in particular for datarelay satellites (DRS) users, for military applications, for the spaceshuttle, or indeed for the destruction command link of the Arianelauncher.

[0014] Spread spectrum modulation is known per se, and is described, forexample, in the following works:

[0015] J. H. Holmes, Coherent spread spectrum systems, and Kamilo Feher,Wireless digital communications (modulation and spread spectrumapplications). Spread spectrum techniques make it possible to define aplurality of logical channels on a single physical channel correspondingto a carrier frequency and to a data passband. The best known of thesetechniques are the following:

[0016] direct sequence spread spectrum which includes code divisionmultiple access (CDMA);

[0017] frequency hopping (FH) spread spectrum which also includes CDMA;and

[0018] carrier sense multiple access (CSMA) spread spectrum.

[0019] In spread spectrum systems, the term “bit” is used for the binaryinformation transmitted over a channel, while the term “chip” is usedfor the binary information associated with the pseudo-random sequence.These techniques make use of pseudo-random codes that are generallyreferred to as pseudo-noise (PN) codes. PN codes are selected usingcriteria associated with their autocorrelation or cross-correlationfunctions, so that a receiver decodes only the signal which is intendedfor that receiver, together with a possible interference signal whichdepends on the number of users and on the properties of the codes.

[0020] In the spread spectrum standard implemented by space agencies fora TTC link, the up channel transmits simultaneously in unbalancedquadrature PSK (UQPSK) both the TC signal spread by a Gold code oflength of 1023 on channel I, and the ranging signal which is a long PNcode (1023*256) on the Q channel; these I and Q channels correspond toresolving a complex signal on two orthogonal axes. The ranging code isthen retransmitted over the telemetry link. Range is deduced therefromby measuring the time delay perceived on the ground for the ranging codeas relayed by the satellite.

[0021] A TTC link with spread spectrum modulation is more robust in theface of interference than is a TTC link with subcarrier modulation;furthermore, the problem of co-localization or the size of the frequencyband needed for such co-localization does not arise in such a crucialmanner: it suffices to allocate different PN codes to the differentsatellites. Nevertheless, a spread spectrum TTC link presents thefollowing drawbacks. Firstly, it is expensive to implement insofar as itrequires a modification to standards that make use of subcarriermodulation; in addition, for the down link, this solution presents thedrawbacks:

[0022] of not enabling the residue of the telemetry carrier to be usedas a telecommunications beacon, as is the case in present-day systems;and

[0023] of not providing an effective solution for co-localizationinsofar as CDMA solutions are very sensitive to balancing the powertransmitted by the various users, and this power balancing can bedifficult to implement on the down link.

[0024] This problem of balancing power in co-localization is lesssensitive on the up link where the ground station can continuouslyadjust the power level it transmits, whereas on the down link, thetransmitted power is constant throughout the lifetime of the satelliteand depends on the design of the satellite; this makes it difficult toensure that an entire fleet of satellites behaves in uniform manner.

[0025] There thus exists a need for a TTC link for satellites which isrobust in the face of interference, which makes co-localizationpossible, which can be implemented easily in existing systems, and whichis also as compatible as possible with existing terrestrial systems.

[0026] Consequently, in an implementation, the present inventionprovides a method of transmitting signals over the telemetry, tracking,and command link of a satellite, the method comprising:

[0027] transmitting command signals and ranging signals using spreadspectrum modulation on the command link from the earth to the satellite;and

[0028] transmitting telemetry signals and ranging signals withsubcarrier modulation over the telemetry link from the satellite to theearth.

[0029] Preferably, the command signals and the ranging signals aretransmitted on the same carrier; likewise, the telemetry signals and theranging signals may be transmitted on the same carrier(s).

[0030] In an implementation, the spread spectrum modulation transmissioncomprises:

[0031] transmitting command signals by spreading using a first code; and

[0032] transmitting a ranging signal by spreading using a second code.

[0033] It is then advantageous for the length of the second code to be amultiple of the length of the first code.

[0034] In another implementation, the transmission with subcarriermodulation comprises modulating the carrier by one or more subcarriersand by a plurality of ranging tones.

[0035] It is also possible for the method to comprise:

[0036] the satellite receiving the ranging signal spread by the secondcode; and

[0037] the satellite modulating the telemetry carrier which is modulatedby the telemetry subcarrier and by a plurality of tones as a function ofthe received ranging signal.

[0038] Preferably, the spread spectrum modulation is UQPSK modulationspread by a Gold code. The subcarrier modulation may be PSK/PMmodulation.

[0039] The invention also provides a satellite presenting:

[0040] a receiver circuit for receiving spread spectrum modulatedcommand signals; and

[0041] a transmitter circuit for transmitting subcarrier modulatedtelemetry signals.

[0042] Advantageously, the receiver circuit presents:

[0043] a receiver channel for receiving command signals spread by afirst spreading code; and

[0044] a receiver channel for receiving ranging signals spread by asecond spreading code.

[0045] In which case, advantageously, the second code presents a lengththat is a multiple of the first code, and the receiver channel forreceiving the ranging signals is controlled by the receiver channel forreceiving the command signals.

[0046] In another embodiment, the transmitter circuit transmits aranging signal. In which case, the satellite may present a link circuitconnecting the receiver circuit to the transmitter circuit, and theranging signal may be transmitted by the transmitter circuit as afunction of signals received over the receiver channel for receivingranging signals.

[0047] Other characteristics and advantages of the invention appear onreading the following description of implementations of the inventiongiven purely by way of example and described with reference to theaccompanying drawing, in which the sole FIGURE is a block diagram of aportion of a satellite constituting an embodiment of the invention.

[0048] For the TTC link of a satellite, the invention proposes usingspread spectrum modulation for the command up link, and it proposesusing subcarrier modulation for the telemetry down link. The use of aspread spectrum command link makes it possible to solve problems ofinterference and limitations in terms of co-localization that arise withsubcarrier modulation; the use of a subcarrier telemetry down link makesit possible to limit the amount of modification required compared withexisting standards: compared with a TTC link having subcarriermodulation for TC and TM, the proposed solution minimizes the costsassociated with changing the standard, insofar as only one of the twolinks is modified. The use of subcarrier modulation on the TM link makesit possible to continue using the residue of the telemetry carrier as atelecommunications beacon as in present-day systems. Finally, the use ofsubcarrier modulation on the TM link makes it possible to usealready-existing terrestrial equipment, and in particular the telemetrylink networks for use in putting the satellite on station.

[0049] For the ranging signals, the invention proposes making use on theup link and on the down link of the modulation and carriers respectivelyused for the command signals and for the telemetry signals.

[0050] The proposed solution goes against the teaching in the state ofthe art, in which TTC links use the same modulation for command and fortelemetry; nor is there anything in the state of the art to suggestcombining different kinds of modulation, particularly since the solutionfor measuring distance on TTC links using spread spectrum modulation aredifferent from those using subcarrier modulation.

[0051] Nevertheless, the proposed solution is viable and advantageous;from the point of view of interference, TC interference is much morethan damaging than TM interference: with TC interference it is possiblethat the satellite will execute the wrong command and the consequencesof that can be very severe. With TM interference, there is a momentaryloss of the observability of the satellite. In addition, a TC link ismuch more subject to interference than is a TM link: the TC coverage ofa satellite on station is in the best of circumstances itscommunications coverage, with it thus being possible to receive a largenumber of interference sources (whether intentional or inadvertent). Incontrast, reception of the telemetry signal usually takes place with ahigh gain antenna on the ground, and thus with a beam of very narrowwidth, consequently providing immunity from any adjacent satelliteswhich might interfere. As a result, using subcarrier modulation on thetelemetry link is a viable solution in spite of the problem ofinterference; the solution proposed above serves to limit theconsequences of interference on the command link, and thus to ensurethat commands are properly transmitted to the satellite.

[0052] Concerning co-localization, the spectrum of the above-mentionedBPSK/FM export command signal is naturally ill-suited to effectivefrequency-division multiple access (FDMA) because of the width of itsspectrum. A BPSK/PM telemetry signal has a narrow spectrum, about 150kHz, and is much more suitable to effective FDMA, particularly since thefiltering for separating TM signals from various co-localized satellitesis performed on the ground where high performance filter techniques canbe implemented at much lower cost than on board a satellite. The use ofspread spectrum modulation on the command link makes it possible toavoid the problem of spectrum width when a plurality of satellites areinvolved; as explained above, this problem is smaller for the telemetrylink since its spectrum is narrower.

[0053] Finally, it is when a satellite is on a drift orbit that the riskof the ground stations for that satellite interfering with some othersatellite is at its greatest: the consequence is a loss ofcommandability so long as the drifting satellite is close to a satellitethat would receive interference. Implementing a spread spectrum commandlink can help solve this problem; where appropriate, the solution may beassociated with deploying the communications antenna on the satellite ona drift orbit so as to be able to reduce the command flux from theground satellite associated therewith.

[0054] A satellite implementing this embodiment of the invention thuspresents a circuit for receiving command signals that are spreadspectrum modulated, and a circuit for transmitting telemetry signalsthat are subcarrier modulated.

[0055] A particular implementation of the invention is described below.In this example, UQPSK modulation is used for the spread spectrummodulation; the modulated bits of the command signal (channel I) arespread by a Gold code having a length of 1023. The chip rate is 500kilocycles per second (kcycles/s). A ranging signal is also transmittedon the command link (on channel Q) and corresponds to a ranging PN codehaving a length of N*1023, i.e. a multiple of 1023. The chip rate isidentical to that used for transmitting command signals. By selectingfor the ranging signal a code that presents a length that is a multipleof the length of the code used for spreading the command signals, it ispossible to simplify synchronization and tracking on the ranging code:once the first code is synchronized, there remain N positions to betested for the long code and not N*1023; naturally, this assumes thatthe relative positions of two codes are known, and for example that thesecond code begins at the same time as the first code.

[0056] The length of the spreading code for the ranging signals isselected as a function of the need to resolve ambiguity; distance ismeasured by measuring the shift due to the propagation of the codes;this measurement is performed modulo the length of the code. A pluralityof possible positions are thus obtained for the satellite, and only oneof them corresponds to the real position of the satellite: the longerthe code, the greater the distance between two potential positions forthe satellite. The length of the code depends on the maximum distanceover which it is necessary to resolve ambiguity, i.e. the distance whichcan be accepted between potential positions for the satellite asprovided by measurement. In practice, and for geostationary satellites,a maximum ambiguity-resolving distance of 5000 kilometers (km) cansuffice; the length of the code is then selected so that the position ofthe satellite obtained by ranging is obtained modulo a distance that isgreater than or equal to 5000 km. In the example of a code having alength which is a multiple of 1023, with a chip rate of 500 kcycles/s, acode of length 17*1023 suffices.

[0057] PSK/PM modulation is used for modulating the subcarrier. Forranging, it is possible, as in the state of the art, to make use ofmodulation of the telemetry carrier simultaneously with modulation ofthe telemetry subcarrier, using tones that are selected in compliancewith the standard proposed by the ESA:

[0058] major tone at 100 kHz;

[0059] virtual minor tones in geometric progression with a ratio of 1/5relative to the major tone (20,000 Hz, 4000 Hz, 800 Hz, 160 Hz, 32 Hz, 8Hz); and

[0060] transmitted minor tones obtained by mixing with other minor tones(20,000 Hz, 16,000 Hz, 16,800 Hz, 16,160 Hz, 16,032 Hz, 16,008 Hz).

[0061] This selection makes it possible to measure distance by measuringthe phase shift of the major tone, with ambiguity being resolved bymeans of the minor tones; the example of a geometric progression with aratio of 1/5 leads to a maximum amount of ambiguity that can be resolvedthat is of the order of 18,000 km. The virtual minor tones are not easyto transmit, particularly those at the lowest frequency. Mixing themwith other tones makes it possible to raise the frequency of the minortones so that they can be transmitted over the down link.

[0062] The FIGURE is a block diagram of a portion of a satellite; theFIGURE shows only that portion of the satellite which serves to receivethe ranging PN code transmitted over the command link and fortransmitting the ranging signal over the telemetry link by modulatingthe carrier and the subcarrier.

[0063] The FIGURE shows firstly a fragmentary view of the command linkreceiver 2. This receiver has a radiofrequency processing system 4 atits input serving to amplify and change the frequency of the signalstransmitted from the earth and received on an antenna (not shown); theamplified or frequency-converted signals are applied to a circuit 6 forsynchronizing and tracking the spreading code used for the commandsignals (the “first” code); in parallel, the amplified signals areapplied to a circuit 8 for synchronizing and tracking the spreading codeused for the ranging signals (or “second” code); as explained above,insofar as the length of the second code is a multiple of the length ofthe first code, and insofar as the length of the chip is the same,synchronization on the first code can serve to simplify synchronizationon the second code. The arrow connecting the synchronizing circuit 6 tothe synchronizing circuit 8 symbolizes this use of synchronization onthe first code to simplify synchronization on the second code. Thereceiver circuit 2 thus presents two channels, a first channel forcommand signals spread by the first code, and a second channel forranging signals spread by the second code.

[0064] The signals delivered by the synchronizing circuit 6 are used inconventional manner for demodulating command signals. A chip counter 10has a count input receiving chip clock signals delivered by thesynchronizing and tracking circuit 8; a reset input of the chip counter10 also receives a signal coming from the synchronizing and trackingcircuit 8. This reset signal is applied to the chip counter by thesynchronizing and tracking circuit at the beginning of the second code.The counter counts up to a number equal to the length of the second codebefore being reset to zero at the beginning of the following code.

[0065] As a result, the chip counter 10 counts the chips of the secondcode as received from the earth over the up link. The output from thechip counter 10 is applied to the addressing input of two programmableread-only memories (PROMs) 12 and 14. Each of these two memories acts asa data table for the ranging tones; more precisely, the PROM 12 containsdata corresponding to the different phases of the major tone, while thePROM 14 contains data corresponding to the different phases of thevirtual minor tones. Digital signals are thus obtained from the readoutputs of the PROMs 12 and 14 that are representative of the major toneand of the minor tones and that are in phase with the chips receivedover the ranging up link.

[0066] The signals coming from the PROMs 12 and 14 are appliedrespectively to digital-to-analog converters (DACs) 16 and 18. The firstDAC thus outputs a major tone that is phase-shifted as a function of thereceived second code; the second DAC thus outputs the virtual minortones occupying the range 8 Hz to 20 kHz in this example. The tonessupplied by the two DACs 16 and 18 are applied to a set of mixers 20.The mixers mix the virtual minor tones with aminor tone at 16 kHz, forexample, so as to output the major tone together with the real minortones as specified above.

[0067] The output from the mixer 20 is applied to a PSK/PM transmitterso as to send the ranging signals simultaneously with the telemetrysignals.

[0068] On arrival, ranging is performed with the major tone; thisprovides measurement accuracy of about 10 meters (m) under nominal linkbudget conditions with standard performance for the on-board and groundequipment and for phase being analyzed with precision of 0.2°;unfortunately, ambiguity is about 1.5 km; the minor tones which are sentsequentially or simultaneously with the major tone serves to resolvethis ambiguity.

[0069] Various possibilities can be envisaged for resolving ambiguity byusing the minor tones; each minor tone may be sent sequentially at anagreed rate, for example the minor tone may be changed at the beginningof each new second code. It is also possible to synchronize minor tonechangeover from the ground: for example it is possible to cease sendingthe second code on the up link and to program the device to change minortone each time there is an interruption in the transmission of thesecond code. It is also possible to use a system of two second codes, afirst of these second codes triggering changes of minor tone, while asecond of these second codes causes the current minor tone to beretained. One way or another, the various minor tones are usedsequentially for the purpose of resolving ambiguity concerning theposition of the satellite.

[0070] The circuit shown in the FIGURE is merely one example of acircuit that enables the tones used on the down link for modulating thetelemetry carrier to be generated as a function of how the second codeis received on the up link. The satellite thus presents a circuit 2 forreceiving spread spectrum modulated command signals, a circuit 22 fortransmitting ranging signals modulating the telemetry carrier, and alink circuit 24 between the receiver 2 and the transmitter 22 andserving to modulate the telemetry carrier as a function of the receivedranging signal. In the example shown in the FIGURE, the link circuit 24comprises a counter 10, data tables 12 and 14, DACs 16 and 18, and themixer 20.

[0071] Naturally, the present invention is not limited to the examplesand implementations described and shown, and it can be varied innumerous ways by the person skilled in the art. Thus, it can beimplemented on a single satellite, in which case the problem ofco-localization clearly does not arise. The proposed solution appliesnot only to the technique of spreading using a direct sequence as givenby way of example, but also to other techniques for spreading spectrum.

[0072] Examples are given above of kinds of modulation that correspondsubstantially to the various standards presently in existence; choosingthese kinds of modulation makes it possible to limit the amount ofmodification that needs to be made to terrestrial and satelliteequipment; it is also possible to use spread spectrum and subcarriermodulation of kinds that are different from those proposed above. Inparticular, it is not essential to use a code for spreading the rangingsignals on the command channel that presents a length that is a multipleof the length of the code for spreading the command signals; this merelymakes synchronization more complicated for the ranging signals.

[0073] The configuration shown in the FIGURE can also be simplified;depending on the chosen number n, a major tone can be delivered merelyby dividing the 500 kHz chip clock and filtering. This major tone couldhave a value other than 100 kHz. The same solution can also be appliedto the minor tones, using different division ratios, and as a functionof the length ratio N between the second code and the first code; itwould also be possible to use phase-locked loops for obtaining the codesused on the down link.

[0074] Various solutions are possible for avoiding the problem of lowfrequencies that arises with the minor tones, other than mixing minortones. It is thus possible to use a subcarrier, e.g. at around 10 kHz to20 kHz that is frequency-modulated by the low frequency minor tones.Like the solution proposed above, this solution makes it possible totransmit the minor tones independently of their frequency.

[0075] Another variant of the circuit shown in the FIGURE consists inreplacing the counter by an accumulator whose function is to sum anincrement value on each clock pulse. The integration clock is the chipclock of the up channel: it is then possible to change the frequency ofthe ranging tone merely by changing the increment of the accumulator.

1/ A method of transmitting signals over the telemetry, tracking, andcommand link of a satellite, the method comprising: transmitting commandsignals and ranging signals using spread spectrum modulation on thecommand link from the earth to the satellite; and transmitting telemetrysignals and ranging signals with subcarrier modulation over thetelemetry link from the satellite to the earth. 2/ The method of claim1, characterized in that the command signals and the ranging signals aretransmitted on the same carrier. 3/ The method of claim 1 or claim 2,characterized in that the telemetry signals and the ranging signals aretransmitted on the same carrier(s). 4/ The method of claim 1, 2, or 3,characterized in that the spread spectrum modulation transmissioncomprises: transmitting command signals by spreading using a first code;and transmitting a ranging signal by spreading using a second code. 5/The method of claim 4, characterized in that the length of the secondcode is a multiple of the length of the first code. 6/ The method of anyone of claims 1 to 5, characterized in that the transmission withsubcarrier modulation comprises modulating the carrier by one or moresubcarriers and by a plurality of ranging tones. 7/ The method of claims4 and 6, characterized in that it comprises: the satellite receiving theranging signal spread by the second code; and the satellite modulatingthe telemetry carrier which is modulated by the telemetry subcarrier andby a plurality of tones as a function of the received ranging signal. 8/The method of any one of claims 1 to 7, characterized in that the spreadspectrum modulation is UQPSK modulation spread by a Gold code. 9/ Themethod of any one of claims 1 to 8, characterized in that the subcarriermodulation is PSK/PM modulation. 10/ A satellite presenting: a receivercircuit (2) for receiving spread spectrum modulated command signals; anda transmitter circuit (22) for transmitting subcarrier modulatedtelemetry signals. 11/ The satellite of claim 1, characterized in thatthe receiver circuit (2) presents: a receiver channel for receivingcommand signals spread by a first spreading code; and a receiver channelfor receiving ranging signals spread by a second spreading code. 12/ Thesatellite of claim 11, characterized in that the second code presents alength that is a multiple of the first code, and in that the receiverchannel for receiving the ranging signals is controlled by the receiverchannel for receiving the command signals. 13/ The satellite of claims10, 11, or 12, characterized in that the transmitter circuit transmits aranging signal. 14/ The satellite of claim 12, characterized in that itpresents a link circuit (24) connecting the receiver circuit (2) to thetransmitter circuit (22), and in that the ranging signal is transmittedby the transmitter circuit as a function of signals received over thereceiver channel for receiving ranging signals.